If you want to run at 4GHz all the time (and expect to have a moderate to high load), and you are having temperatures around 90, you really will need to consider better cooling (or a lower overclock). Yes, I suppose 90 is ok, as long as you don't plan on folding 24/7, or doing other high load tasks. If you are concerned about it getting too hot, I would recommend setting overheat protection in Core Temp.